Error: unknown type name 'FILE

Question

I am making a function that simply writes "hello" to a file. I put it in another file and included its header in the program. But gcc gives an error, namely: error: unknown type name 'FILE. The code is below

app.c:

#include<stdio.h> #include<stdlib.h> #include"write_hello.h" int main(){ FILE* fp; fp = fopen("new_file.txt","w"); write_hello(fp); return 0; } 

write_hello.h:

 void write_hello(FILE*); 

write_hello.c:

 void write_hello(FILE* fp){ fprintf(fp,"hello"); printf("Done\n"); } 

when compiling with gcc, the following happens:

 harsh@harsh-Inspiron-3558 :~/c/bank_management/include/test$ sudo gcc app.c write_hello.c -o app write_hello.c:3:18: error: unknown type name 'FILE' void write_hello(FILE* fp){ ^ 

Sorry for any errors. I start.

Answer 1

FILE is defined in stdio.h, and you need to include it in every file that uses it. Therefore, write_hello.h and write_hello.c should both include it, and write_hello.c should also include write_hello.h (since it implements the function defined in write_hello.h).

Also note that it is standard practice for each header file to define a macro with the same name (IN CAPS) and enclose the entire header between #ifndef, #endif. In C, this prevents getting the header twice. This is known as the “internal guard” (thanks to Story Teller for pointing this out).

write_hello.h

 #ifndef WRITE_HELLO_H #define WRITE_HELLO_H #include <stdio.h> void write_hello(FILE*); #endif 

write_hello.c

 #include <stdio.h> #include "write_hello.h" void write_hello(FILE* fp){ fprintf(fp,"hello"); printf("Done\n"); }